The invention relates to a resonator; and more particularly the invention relates to a unitary dual frequency side branch resonator having an inlet branch, a discharge branch, and a resonator branch with attenuating means for attenuating sound waves at a first frequency located in the resonator branch and a resonator branch closed end downstream of the attenuating means for attenuating sound waves at a second frequency.
Mobile temperature control units are typically mounted on the end of a trailer behind the cab. The units can be quite noisy approaching sound levels of 80 dB and the loud units make it difficult for the driver to sleep while the unit is running, and furthermore while driving, the emitted noise can be a source of driver discomfort on long hauling trips. The relatively low driver retention rate in the trucking industry is in large part attributed to relatively high noise emission levels of refrigeration units. Additionally, the relatively loud units introduce considerable noise into the "community" when the units are being unloaded at loading docks, grocery stores, distribution centers or dairies; or when the associated trucks are parked at motels or hotels.
Much of the noise produced by a refrigeration unit is generated by the refrigeration unit's prime mover which is typically a diesel engine. The diesel engine drives a compressor which compresses a conventional refrigerant during a well known conventional refrigeration cycle. A large portion of the diesel engine noise is generated during the combustion process. A portion of the combustion noise produced by the diesel engine flows unabated upstream and out the diesel engine air intake manifold.
Frequently an air cleaner is flow connected to the intake manifold and the air cleaner serves to attenuate the higher frequency wave components of the engine noise. As a result, the resultant filtered engine noise is comprised mainly of low frequency noise. The resultant low frequency noise is at a frequency that is too low to be attenuated by common techniques and methods such as acoustical foam.
Known conventional apparatus for attenuating multiple frequency sound waves are typically expensive and complex and are comprised of multiple component parts such as valves or flappers. Others require separate branches for each frequency sound wave attenuated. Such known devices for attenuating multiple frequencies are bulky and do not easily fit in the limited space of a refrigeration unit.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.